Ta có: \(2x^2+x+1\)
\(=\left(\sqrt{2}x\right)^2+2.\sqrt{2}x.\frac{1}{2\sqrt{2}}+\frac{1}{8}+\frac{7}{8}\)
\(=\left(\sqrt{2}x+\frac{1}{2\sqrt{2}}\right)^2+\frac{7}{8}\ge\frac{7}{8}\)
\(\frac{\Rightarrow\left(\sqrt{2}x+\frac{1}{2\sqrt{2}}\right)^2+\frac{7}{8}}{-2}\le\frac{-7}{16}\)
(Dấu "="\(\Leftrightarrow\sqrt{2}x+\frac{1}{2\sqrt{2}}=0\Leftrightarrow x=\frac{-1}{4}\)
\(D=\frac{2x^2+x+1}{-2}\)
\(=\frac{2\left(x^2+\frac{1}{2}x+\frac{1}{2}\right)}{-2}\)
\(=\frac{2\left(x^2+2.x.\frac{1}{4}+\frac{1}{16}-\frac{1}{16}+\frac{1}{2}\right)}{-2}\)
\(=\frac{2\left(x+\frac{1}{2}\right)^2+\frac{7}{8}}{-2}\)
Vì \(2\left(x+\frac{1}{2}\right)^2\ge0;\forall x\)
\(\Rightarrow2\left(x+\frac{1}{2}\right)^2+\frac{7}{8}\ge\frac{7}{8};\forall x\)
\(\Rightarrow\frac{2\left(x+\frac{1}{2}\right)^2+\frac{7}{8}}{-2}\ge\frac{-7}{16};\forall x\)
Dấu'="xảy ra \(\Leftrightarrow\left(x+\frac{1}{2}\right)^2=0\)
\(\Leftrightarrow x=\frac{-1}{2}\)
Vậy \(D_{min}=\frac{-7}{16}\)\(\Leftrightarrow x=\frac{-1}{2}\)
Lê Tài Bảo ChâuSai quá sai, tại x = 1/2 thì D khác -7/16
ukmc ảm ơn bài mình sai rồi xl
